make windows code built with clangcl include the _posix source code.  depot_tools excludes these source files now, so they need to be manually included.
BUG=435
TESTED=clangcl local build on windows
R=tpsiaki@google.com

Review URL: https://webrtc-codereview.appspot.com/46199004

git-svn-id: http://libyuv.googlecode.com/svn/trunk@1394 16f28f9a-4ce2-e073-06de-1de4eb20be90